Computational studies of emission sources in direct fuel injection natural gas engines

The influences of different fuel injection strategies on emission characteristics in a glow plug assisted direct-injection natural gas engine were computationally investigated by a KIVA-3V engine model. A detailed chemical kinetic model and a phenomenological soot model were implemented in the simulation to predict the formation of engine emissions. The simulation reveals that the sac volume of the fuel injector is one of the major contributors to the methane emissions, and the use of a bigger injector nozzle and shorter injection duration can reduce the fuel storage inside the sac volume. In addition, the glow plug shield will trap some fuel mixture and induce relatively rich combustion that contributes a significant amount to both of methane emission and soot particles. Additional openings in the upper part of the glow plug shield may be needed to improve the flow exchange between the shield and the combustion chamber, in order to reduce the emission formation. Further simulation suggests that different fuel injection strategies, such as changing the injection angle and using different injector nozzle sizes associated with different injection durations, could affect the fuel injection, flame propagation and then emission formation inside the glow plug shield and combustion chamber. An optimized fuel injection strategy thereby has potential of reducing the local emission formation inside the glow plug shield and its contribution to the final combustion emissions.